The present invention relates to a method for the preparation of silicon carbide fibers or, more particularly, to a method for the preparation of silicon carbide fibers by the vapor-phase pyrolysis of an organosilicon compound on a substrate surface.
As is known, silicon carbide fibers, i.e. a fibrous material mainly composed of silicon carbide SiC, are very promising as a material used in high-performance composites with various kinds of metals, ceramics and organic resins by virtue of the outstandingly high heat resistance and oxidation resistance as well as excellent mechanical strengths. Accordingly, several methods have been proposed in the prior art for the preparation of silicon carbide fibers.
Conventional methods for the preparation of silicon carbide fibers are typically classified into three classes. First of the classes is the so-called CVD (chemical vapor deposition) method in which a gaseous mixture composed of a gaseous halogen-containing silicon compound such as silicon tetrachloride SiCl.sub.4, trichlorosilane HSiCl.sub.3 and organochlorosilanes represented by the general formula R.sub.n SiCl.sub.4-n, in which R is a monovalent hydrocarbon group and n is an integer of 1, 2 or 3, and a gaseous hydrocarbon compound such as methane, ethane and the like is pyrolyzed at a high temperature so that the silicon carbide as the reaction product is deposited on the surface of a substrate in the form of whiskers.
The method belonging to the second class of the methods for the preparation of silicon carbide fibers comprises spinning of a polycarbosilane, i.e. an organosilicon polymer composed of a backbone structure of recurring carbon-to-silicon linkages, into filaments followed by firing of them at a high temperature to be converted into long filaments of silicon carbide (see, for example, Japanese Patent Kokai No. 52-70122).
The third class of the methods involves vapor-phase pyrolyzing deposition of free silicon on the surface of carbon fibers followed by firing of the thus silicon-clad carbon fibers at 1400.degree. to 2600.degree. C. to effect the reaction between the carbon and silicon to form silicon carbide in the form of fibers (see, for example, Japanese Patent Kokai No. 50-38700).
These prior art methods have their own respective disadvantages accompanying the advantages and none of the conventional methods is quite satisfactory in one or other respects. For example, the first class method is disadvantageous because the silicon carbide fibers obtained in this CVD method are always whisker-like with limited fiber lengths so that the reinforcing effect obtained in a composite material with such whisker-like silicon carbide fibers is sometimes insufficient even though the reaction temperature of pyrolysis in the range of 1300.degree. to 1500.degree. C. is not excessively high.
The second class method of firing of polycarbosilane filaments is suitable when long filaments of silicon carbide are desired. This method is defective, however, because the method is performed in a very complicated and lengthy process including synthesis of the polycarbosilane polymer as the starting material, spinning of the polymer into filaments and firing of the filaments at high temperatures so that the cost for the preparation of the silicon carbide fibers is necessarily very high.
The third class method is performed in two steps of first depositing silicon on the carbon fibers and then reacting the silicon and carbon by firing the silicon-clad carbon fibers at a temperature of 1400.degree. to 2600.degree. C. so that this method is also disadvantageous in the high production cost as a result of the complicated process involving high temperature treatment.